1. Field of the Invention
This invention relates to a zoom lens and an image projection apparatus having the same, and is suitable for a liquid crystal projector having, for example, a long back focal length, and having high optical performance at various projection magnifications.
2. Related Background Art
There have heretofore been proposed various liquid crystal projectors (image projection apparatuses) using a display element such as a liquid crystal display element to project an image formed on the display element onto a screen surface.
Characteristics mentioned below are required of a projection lens for use in these liquid crystal projectors.
In color liquid crystal projection of a three-element type using three liquid crystal display elements, generally, light from a white source is separated into red, green and blue colors by a color separating optical system, and these color lights are directed to the respective liquid crystal display elements, and the lights emerging from those liquid crystal display elements are combined by a color combining optical system and are made to be incident on a projection lens.
In that projection, the projection lens must have a back focal length of a certain constant length so as to provide a space between each liquid crystal display element and the projection lens for disposing therein a prism or the like for combining the color lights after transmitted through the liquid crystal display elements.
If the angles of the light beams incident from the liquid crystal display elements onto the color combining optical systems change, the spectral transmittance of the color combining optical system changes in conformity therewith, and the brightness of each color in the projected image is changed depending on the angle of view and the image becomes an image difficult to see. Therefore, in order to lessen the influence of the dependence on the angle, the color combining optical system must be a so-called telecentric optical system of which the pupil on the liquid crystal display element (reduction conjugate surface) side is substantially at infinity.
When the pictures (images) of the liquid crystal display elements of the three colors are combined and projected onto a screen, pixels of the respective colors must be superposed one upon another on the entire area of the screen so that high resolution feeling may not be spoiled by, for example, doubly displaying characters or the like.
Therefore, color misregistration (chromatic aberration of magnification) occurring in the projection lens must be well corrected in a visible light zone.
Distortion must be well corrected so that the projected image may not be distorted and difficult to see.
The projection lens must be a bright projection lens of small Fno (F number) so as to be capable of efficiently introducing the light from the light source.
The projector carrying a compact liquid crystal panel thereon must be compact and light in weight to attach importance to portability and mobility.
A good projected image must be obtained at various projection distances, that is, the aberration fluctuation during focusing must be small.
As a zoom lens adapted to satisfy these requirements, there has been a zoom lens for a projector of which the focusing is effected with only some of optical elements in lens units moved (Japanese Patent Application Laid-open No. H10-186235, U.S. Pat. No. 6,580,564 and Japanese Patent Application Laid-open No. 2004-226803).
Japanese Patent Application Laid-open No. H10-186235 discloses a five-unit zoom lens comprising, in succession from a screen side, lens units of negative, positive, positive, negative and positive refractive power and in which the rearmost (projected image side) positive lens in a first lens unit comprising four positive, negative, negative and positive lenses is fixed and three lenses on the screen side are moved to thereby effect focusing.
U.S. Pat. No. 6,580,564 and Japanese Patent Application Laid-open No. 2004-226803 disclose a five-unit zoom lens comprising, in succession from a screen side, lens units of negative, positive, positive, negative and positive refractive power and in which a first lens unit is composed of two lens elements having negative refractive power, and the interval between these lens elements is changed to thereby effect focusing. Both of these are such that a positive lens is included in a focusing element and weakens the refractive power of the entire focusing element.
Also, as a projection lens for a liquid crystal projector, there is proposed a six-unit zoom lens comprising, in succession from an enlargement conjugate side (this term is interchangeably used herein in the same meaning as a front side and an enlarging side), six lens units as a whole by the arrangement of first to sixth lens units of negative, positive, positive, negative, positive (or negative) and positive refractive power, and in which a predetermined lens unit of them is appropriately moved to thereby effect zooming (U.S. Publication No. US-2001-050818).
In this six-unit zoom lens, during zooming from a wide angle end to a telephoto end, in order that with the first and sixth lens units as being fixed, the second to fifth lens units therebetween may all be moved to a reduction conjugate side (this term is interchangeably used herein in the same meaning as a rear side and a reduction side), the full length of the lens is kept constant during zooming. Also, this zoom lens is a zoom lens in which distortion and chromatic aberration during zooming are lessened and which is telecentric on the reduction conjugate side. In the configuration disclosed in this U.S. Publication No. US-2001-050818, there is disclosed that the first lens unit is composed of three positive, negative and negative lenses from the enlargement conjugate side, and this first lens unit is driven to thereby effect focus adjustment.
Generally, as a method of suppressing the fluctuations of various aberrations when focusing is effected, there is a method of strengthening the refractive power of a focusing lens unit as far as possible, and lessening the movement amount of the focusing lens to the utmost. In the aforementioned Japanese Patent Application Laid-open No. H10-186235, U.S. Pat. No. 6,580,564 and Japanese Patent Application Laid-open No. 2004-226803, in the lens construction in the first lens unit, the focusing lens unit is not composed of only negative lenses and therefore, there has been the tendency that the refractive power of the focusing lens unit becomes weak, and the movement amount thereof during focusing becomes great and the fluctuations of various aberrations become great.
Further, it is often the case with the lens construction of a zoom lens used in a projector that it is made into a retrofocus type in order to obtain a long back focal length, and lens units having strong positive refractive power are disposed on a reduction side in order to make the reduction side telecentric.
The zoom lens of such a construction, however, has the tendency that the asymmetry of the entire lens system increases for example, as chromatic aberration of magnification in blue occurs more under (direction of an optical axis) than that in green.
Also, the smaller becomes the number of lenses, the more increases the refractive power of each lens and therefore, particularly a negative lens located more adjacent to the reduction side than a stop becomes liable to cause chromatic aberration of magnification of a high order in the over direction. Also, the greater becomes the zoom ratio, the greater the fluctuation of chromatic aberration of magnification from the wide angle end to the telephoto end also tends to become.